The present invention relates to the communication of digital data using trellis coded amplitude modulation (AM) and trellis coded quadrature amplitude modulation (QAM) with punctured convolutional codes. One of the various applications for which the present invention is particularly well suited is in the transmission of digital television signals.
Digital data, for example, digitized, compressed television (NTSC) or high-definition television (HDTV) signals can be transmitted over terrestrial very high frequency (VHF), ultra-high-frequency (UHF), or cable television analog channels to end users. Analog channels deliver corrupted and transformed versions of their input waveforms. Corruptions of the waveform include linear, frequency-selective amplitude and phase distortion, nonlinear or harmonic distortion, and multiplicative fading. Additive corruption of the waveform, due to statistical thermal and impulse noise, may be countered using forward error correction codes.
In order to communicate digital data via an analog channel, the data is modulated using, for example, a form of pulse amplitude modulation (PAM). Typically, quadrature amplitude modulation or single-sideband (SSB) modulation is chosen to efficiently use the available channel bandwidth. QAM is a quadrature, or orthogonal combination of two PAM signals. When viewed as coordinates of a plane, the combined PAM signals form a "constellation" of possible transmission levels. Each transmitted constellation point is called a symbol. For example, two independent, quadrature four-level AM signals form a 16-QAM constellation which encodes four bits. A 32-point constellation can be formed with dependent six-level AM quadrature signals, encoding five bits per symbol.
In pulse amplitude modulation, each signal is a pulse whose amplitude level is selected from a fixed set of levels. In 16-QAM, each of the quadrature PAM signals select from uniformly spaced, bipolar amplitudes scaled from amplitude levels -3, -1, 1, 3. Spectral efficiency in digital communication systems is defined as the number of transmitted information bits per second per unit of bandwidth, i.e., the ratio of the data rate to the bandwidth. Modulation systems with very high bandwidth efficiency are employed in applications that require high data throughput with small available bandwidth. QAM and SSB provide bandwidth efficient modulation, which can provide very low bit error rates when used with high efficiency forward error correction codes such as trellis coded modulation (TCM).
Trellis coded modulation has evolved as a combined coding and modulation technique for digital transmission over band limited channels. Unlike traditional application of convolutional codes to two-level PAM which increase the bandwidth used in transmission, TCM increases the constellation size instead. In TCM schemes, a sequence of "coded" bits are convolutionally encoded into a sequence of groups which partition the symbol constellation. For each encoded group, a number of "uncoded" bits are transmitted by selecting a unique constellation element of the group. At a receiver, the sequence of transmitted groups is decoded by a soft-decision maximum likelihood (ML) convolutional code decoder. Such TCM schemes can improve the robustness of digital transmission against additive noise by three to six dB or more, compared to uncoded modulation at the same information rate.
Most TCM schemes map one step of the convolutional code trellis to one transmission symbol which consists of two QAM components (I, Q). Such two-dimensional (2-D) codes achieve a throughput of an integer number of information bits per 2-D symbol. It is desirable to increase the TCM throughput by increasing the integer number of coded bits per symbol by some fraction. Schemes have evolved which combine two 2-D symbols to form 4-D symbols, four 2-D symbols to form 8-D symbols, and so on, to obtain fractionally higher throughputs. These "multi-dimensional" codes achieve higher spectral efficiencies at the cost of much increased decoder complexity. Such complexity results from the need to compute soft-decisions of each multi-dimensional group within the constellation and the need to build a custom convolutional decoder of a rate n/k code, where n/k represents the fractional throughput to be provided.
In many data communications applications requiring a very low probability of bit error, a concatenation of two forward error correction codes is often used. An "inner" soft-decision code is used on the noisy channel to deliver a modest symbol error rate to an "outer" decoder. A known approach is to use a convolutional or trellis code as the inner code with some form of the "Viterbi algorithm" as a trellis decoder. The outer code is most often a t-symbol correcting "Reed-Solomon" or other algebraic block code. Such codes are well known in the art. The outer decoder removes the vast majority of symbol errors that have eluded the inner decoder in such a way that the final output error rate is extremely small.
With concatenated coding, the inner code typically needs to provide only three to four dB of coding gain, before sending the partially corrected data to the outer code. Multi-dimensional codes, which achieve higher throughput rates than 2-D trellis codes, are designed to approach six dB coding gain at high signal-to-noise ratios (SNR).
It would be advantageous to be able to achieve the spectral efficiencies of multi-dimensional codes without having to compute the soft-decisions of a plurality of multi-dimensional groups, and without having to build a convolutional decoder of a rate n/k instead of a rate 1/m. It is known that high rate n/k convolutional codes can be implemented for traditional two-level modulation (e.g., BPSK) with bandwidth expansion using punctured rate 1/m codes. However, application of punctured convolutional codes to nonbandwidth expanding TCM has only recently been considered. One such scheme is disclosed in commonly assigned, copending U.S. patent application Ser. No. 07/912,542 filed on Jul. 13, 1992 for "Apparatus and Method for Communicating Digital Data Using Trellis Coded QAM with Punctured Convolutional Codes," incorporated herein by reference.
The present invention provides a communication scheme that enjoys the aforementioned advantages. In particular, the present invention takes advantage of the reduced coding gain requirement for TCM when concatenated with an outer code. The invention also applies punctured convolutional codes to TCM by puncturing a standard rate 1/2 convolutional decoder to achieve fractional rate n/k throughputs, where n/k can be any desired value less than one.